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Article

Performance of Dye-Containing Wastewater Treatment Using MnxOy-Catalyzed Persulfate Oxidation

by
Yujuan Li
1,
He Guo
2,
Hu Li
3,* and
Tiecheng Wang
4,*
1
Ningxia Academy of Environmental Sciences, Co., Ltd., Yinchuan 750000, China
2
College of Biology and the Environment, Nanjing Forestry University, Nanjing 210037, China
3
School of Ecology and Environment, Ningxia University, Yinchuan 750021, China
4
College of Natural Resources and Environment, Northwest A&F University, Yangling 712100, China
*
Authors to whom correspondence should be addressed.
Catalysts 2024, 14(11), 758; https://doi.org/10.3390/catal14110758
Submission received: 25 September 2024 / Revised: 17 October 2024 / Accepted: 25 October 2024 / Published: 27 October 2024
(This article belongs to the Section Environmental Catalysis)
Browse Figures
Versions Notes

Abstract

:
Dye wastewater is characterized by high salinity, intense coloration, difficulty in degradation, and complex organic compositions, posing significant environmental risks. Manganese oxide (MnxOy)-based materials have been widely used for the removal of recalcitrant organic pollutants in water environments. In this study, various MnxOy polymorphs were prepared, and their catalytic activities for persulfate (PS) activation were evaluated using Orange II (AO7) as a model molecule. After 50 min treatment, the degradation efficiency of AO7 ranked as α-MnO2/PS > γ-MnO2/PS > β-MnO2/PS > Mn2O3/PS, with α-MnO2/PS achieving the highest efficiency of 98.6%. XPS, XRD, and electrochemical analyses indicated that α-MnO2 exhibited an exceptional crystal structure and performance. The α-MnO2/PS system exhibited a strong pH adaptability across a wide pH range of 3.0–9.0. The presence of coexisting anions at 0.1 mM, including Cl−, NO3−, CO32−, and SO42−, slightly reduced the degradation rate of AO7. The reactive oxygen species, mainly SO4•− and 1O2, predominantly destroyed the naphthalene ring structure of AO7. Furthermore, α-MnO2 exhibited an excellent stability, allowing for multiple reuse cycles without interference from common anions in water, highlighting its strong potential for practical applications. These results provided insights into the environmental fates of AO7 in the α-MnO2/PS system.

1. Introduction

With the rapid advancement of science, technology, and industry, large volumes of industrial wastewater containing organic dyes from sectors such as textiles, printing, papermaking, and cosmetics are discharged into the environment without proper treatment, making it a major contributor to current water pollution [1,2]. Nearly 100,000 types of dyes are commercially synthesized, with global consumption reaching around 1.6 million tons and resulting in approximately 4.8 billion tons of annual emissions [3]. Among these, Orange II (AO7) is an azo dye widely used for dyeing wool, silk, and leather. It is known for its complex composition, resistance to degradation, high discharge volumes, and vivid coloration [2,4]. AO7 poses significant risks to environmental sustainability and human health due to its toxicity, mutagenicity, and carcinogenicity [2,5]. With escalating water scarcity, treating dye-containing wastewater has become a pressing environmental challenge that impedes sustainable development [6]. Effective methods for dye removal and treatment are urgently needed.
Traditional methods for dye removal include adsorption [7], coagulation [8], and membrane filtration [9]. Adsorption is effective for both the pretreatment and deep treatment of organic dye wastewater, but recycling adsorbents is challenging [7]. Coagulation generates substantial sludge and is highly dependent on pH, making it unsuitable for acidic and azo dye wastewater [8]. Membrane filtration effectiveness is influenced by membrane characteristics and is hindered by high costs and fouling issues [9]. As dye wastewater volumes increase, there is an urgent need for more efficient and rapid treatment methods. Advanced oxidation processes (AOPs), such as photocatalysis [10], electrochemical oxidation [11,12], and Fenton oxidation [13,14], have been developed as alternatives.
Persulfate-based AOPs offer promising solutions due to their convenient storage, broad pH operating range (3–11), and potent sulfate radical (SO4•−) oxidation power [15,16]. Despite their effectiveness, persulfate alone has weak oxidation capacity and requires activation [17,18]. Transition metal-based heterogeneous catalysts are particularly advantageous for this activation, offering zero energy consumption, broad pH adaptability, and scalability [19,20]. Metals such as Co, Fe, Cu, Mn, and La can promote persulfate decomposition to generate reactive oxygen species (ROS), which are effective in eliminating persistent organic pollutants, as evidenced by Equations (1) and (2) [21,22,23]. Manganese (Mn), abundant in the environment, particularly in soils and sediments, [24] is known for its strong redox cycling capabilities and excellent oxygen migration rates [25,26]. Thus, it is recognized as a promising material to remove organic contaminants in water treatment. MnO2, with its diverse crystalline forms (e.g., λ-MnO2, β-MnO2, α-MnO2), is widely used in oxidation techniques for organic pollutant degradation due to its low cost, excellent catalytic properties, and environmental friendliness [27,28,29]. Yan et al. [30] found that the CuO/α-MnO2 bimetallic catalyst effectively degrades thiocyanate (SCN−) in aqueous solutions. The calcination temperature significantly influences its catalytic performance. Shi et al. [31]. examined the interfacial reactions between MnO2 and dissolved organic matter (DOM) of different molecular weights, revealing significant differences in their interactions with MnO2. Previous research indicates that the various crystal phases of MnO2 directly affect physicochemical properties, such as surface atomic distribution and oxygen vacancy formation, thereby influencing its reactivity [29,32,33,34]. Nonetheless, the mechanisms underlying the differences in ROS generation and AO7 degradation during PMS activation by various MnO2 crystals remain unclear.
HSO5− + Mn + → SO4•− + OH− + M(N+1)+
HSO5− + Mn + → SO42− + •OH + M(N+1)+
In this study, various MnO2 polymorphs (α-MnO2, β-MnO2, γ-MnO2, and Mn2O3), were synthesized and their catalytic performances were evaluated through AO7 degradation experiments. First, the chemical structures, catalytic properties, and electron transfer capacity of MnO2 polymorphs were characterized. Then, single-factor experiments were conducted to explore the effects of catalyst dosage, oxidant concentration, initial solution pH, and coexisting anions on the catalytic degradation performance of the MnxOy/PS system. Subsequently, the degradation mechanism of AO7 in the MnxOy/PMS system was investigated using radical quenching experiments, full-spectrum UV-vis spectroscopy, and electrochemical measurements. Finally, the stability and reusability of α-MnO2 polymorphs were evaluated.

2. Results and Discussion

2.1. Degradation Performance of AO7 at Various MnxOy Polymorphs

Figure 1 illustrates the removal efficiency of AO7 using α-MnO2, β-MnO2, γ-MnO2, and Mn2O3, as well as persulfate (PS) alone. PS alone did not effectively degrade AO7. When 0.05 mol/L of PS and MnxOy were both added, the degradation efficiencies of AO7 within 50 min achieved 98.60% for α-MnO2, 82.47% for β-MnO2, 97.15% for γ-MnO2, and 16.14% for Mn2O3 (Figure 1a). The pseudo-first-order kinetic model was applied to the degradation curves of AO7 for the different MnxOy polymorphs (Figure 1b). The observed rate constant kobs was 0.086 min−1 for α-MnO2/PS, followed by 0.079 min−1 for γ-MnO2/PS, 0.037 min−1 for β-MnO2/PS, and 0.001 min−1 for Mn2O3/PS (Figure 1b). The reaction kinetics constant reflects the relationship between the reaction rate and the concentration of reactants; a larger constant indicates a faster reaction rate, suggesting that the reaction proceeds more easily [32,33,34]. These results indicated that α-MnO2, γ-MnO2, and β-MnO2 were effective in removing AO7, while Mn2O3 showed a weak catalytic capacity. Previous studies also suggested that Mn2O3 had low catalytic activities for persulfate activation [26,27]. The variability in dye degradation performance among different MnxOy polycrystals mainly arises from the diverse mechanisms involved, such as redox reactions, surface adsorption, free radical generation, crystal phase and facet effects, and electron transfer [31]. Manganese in various oxidation states generates reactive oxygen species through redox cycling, promoting dye degradation [20,21]. Changes in crystal phase structures and surface active sites further influence the catalytic efficiency. Electron conductivity and oxygen vacancy concentration both impact free radical formation, while solution pH regulates the oxidative activity of manganese oxides, together determining dye degradation efficiency [21,32].
Additionally, the adsorption properties of catalytic materials played a crucial role in pollutant removal. Figure 1c depicts the adsorption efficiency for AO7 of the four catalytic materials. The adsorption effect followed the order γ-MnO2 > α-MnO2 > β-MnO2 > Mn2O3. In the absence of PS, MnxOy polycrystals (except for Mn2O3) exhibited some adsorption of AO7, which aligned with their removal efficiencies. γ-MnO2 showed the highest adsorption due to its densely packed hexagonal structure, which alternated between pyrolusite (1 × 1) and romanechite (1 × 2) tunnels [25,35]. This structure introduced numerous defects and vacancies, enhancing its adsorption properties [24,31]. Previous study found that dye degradation starts with adsorption onto the catalyst via electrostatic, van der Waals, and hydrogen bonding interactions [29]. Active sites like Mn4+ and Mn3+ then facilitate redox reactions, breaking down the dye into non-toxic substances [21,29]. In summary, α-MnO2 exhibited the highest efficiency for AO7 removal among the tested MnxOy materials.

2.2. Characterization of the MnxOy Polymorphs

The electrochemical properties of catalytic materials significantly influenced their pollutant removal capabilities [36]. In our study, the electrochemical performances of various MnxOy polycrystals were systematically evaluated, revealing that α-MnO2 exhibited superior electron transfer capabilities compared to other polymorphs. This was demonstrated by the highest current response observed in the cyclic voltammetry (CV) curves (Figure 2a), indicating the great electron mobility of α-MnO2. Furthermore, the linear sweep voltammetry (LSV) results (Figure 2b) revealed that α-MnO2 facilitated faster catalytic reaction kinetics and superior catalytic performance compared to other MnxOy polymorphs. Additionally, the electrochemical impedance spectroscopy (EIS) analysis (Figure 2c) showed that α-MnO2 possessed a lower charge transfer resistance, underscoring its efficiency in electron transfer as compared to γ-MnO2, β-MnO2, and Mn2O3.
The crystallographic structures of the MnxOy polymorphs were examined via X-ray diffraction (XRD), with patterns presented in Figure 3a. The diffraction peaks at 2θ values of 13.3°, 18.3°, 30.4°, 37.5°, 42.9°, 49.8°, and 62.3° correspond to the (110), (200), (211), (301), (411), (600), and (521) planes of MnO2, matching the reference pattern (PDF#44-0141). This comparison confirmed the successful synthesis of α-MnO2, attributable to the bonding motifs of [MnO6] octahedral units [24,25]. Surface chemical composition analysis via X-ray photoelectron spectroscopy (XPS) revealed that manganese (Mn) was the predominant element within the MnO2 polymorphs (Figure 3b). High-resolution XPS spectra identified peaks associated with Mn2+ at binding energies of 642.3 eV and 653.7 eV, Mn3+ at binding energies of 643.2 eV and 654.6 eV, and Mn4+ at binding energies of 644.3 eV and 655.2 eV (Figure 3c). The Mn3+/Mn4+ ratio on the surface of α-MnO2 was calculated to be 1.33, indicating a higher concentration of Mn3+, which facilitated the cleavage of O−O bonds in persulfate (PS), enhancing single-electron transfer and accelerating redox reactions [31]. In addition, this process accelerated the exchange between O22−/OH and increased the rate of redox reactions in the system, hereby improving pollutant removal efficiency [37]. Previous studies have shown that variations in crystal structure significantly affect catalytic performance through several mechanisms, such as the regulation of active site distribution, electron transfer pathways, oxygen vacancy concentration, and reactant adsorption capacity. Moreover, variations in the stability of different crystal phases can impact the lifespan of the catalyst, thereby collectively influencing its efficiency and durability in redox reactions [20,21,29].
The O 1s spectrum of XPS is depicted in Figure 3d, displaying peaks corresponding to lattice oxygen (Olatt) and surface-adsorbed oxygen (Oads). Specifically, the peak at 529.9 eV could be attributed to Olatt (Bi–O) within the MnO2 structure, while the peak at 531.6 eV originated from Oads adsorbed on the material surface (O–H), and the Oads/Olatt value of α-MnO2 surface was 0.34 [38,39]. The presence of Oads indicated the existence of low-coordination oxygen defect sites, namely oxygen vacancies (O vacancies) on the material surface [39,40]. In the chemical or physical structure of materials, the presence of oxygen vacancies could significantly affect their electronic structures and chemical properties [39]. Furthermore, based on their 3D structural analysis of diffusion energy pathways in different crystals, Fu and Luo found that α-MnO2 exhibits the highest binding energy and the second-lowest diffusion barrier compared to γ-MnO2 and β-MnO2 [41,42].
In summary, α-MnO2 exhibits strong oxidative capacity and high binding energy, resulting in significant efficiency in its reaction with persulfate. Through this reaction, α-MnO2 can generate a series of reactive radicals, including SO4•− and •OH, which effectively attack chemical bonds within organic molecules, leading to the efficient removal of dyes and other contaminants [20,38]. Moreover, the high oxidation state of α-MnO2 enhances its interaction with persulfate, resulting in a substantial increase in reaction rate.

2.3. AO7 Degradation in Different Conditions

Figure 4a shows the degradation performance of AO7 with treatment time under various α-MnO2 dosages, and their evolution kinetics is depicted in Figure 4b. It was observed that α-MnO2 dosage markedly enhanced the degradation efficiency of AO7. Under the PMS concentration of 0.05 mol/L and the reaction time of 30 min, the degradation efficiencies of AO7 with α-MnO2 dosages of 0 g, 0.025 g, 0.050 g, and 0.075 g were 12.10%, 90.80%, 98.3%, and 99.7%, respectively. The reaction rate constant (k) increased from 0.075 to 0.208 min−1 as the α-MnO2 dosage increased, indicating that more active sites were available for catalyzing PS to generate transient active species. Excessive α-MnO2 (0.10 g) led to a decrease in degradation efficiency due to possible interference or competition between active sites and aggregation of catalyst particles, which reduced the effective surface area and available active sites [39,43]. Additionally, excess catalyst may induce side reactions that consumed active species, further diminishing AO7 degradation efficiency [31].
Increasing the PS concentration accelerated the reaction rate and promoted the generation of reactive oxygen species (ROS), which aided in the degradation of organic pollutants (Figure 4c,d). The degradation rate of AO7 increased with rising PS concentration. As the PS concentration increased from 0.005 mol/L to 0.2 mol/L, the degradation efficiency of AO7 rose from 80.5% to 98.3% after 30 min, indicating a positive correlation between PS concentration and degradation efficiency. Higher concentrations of PS enhanced the interaction with α-MnO2 by increasing the number of PS molecules available for contact with the catalyst surface, thereby improving electron transfer efficiency, activating PS more effectively, generating more reactive oxygen species, and subsequently accelerating the oxidation reactions and pollutant degradation [28].
The solution pH also played a crucial role in ROS generation and the types of organic compounds present in the aqueous phase [44]. Figure 4d,e demonstrate that AO7 degradation rates under acidic, neutral, and weakly alkaline conditions consistently exceed 98.0% after 30 min, indicating that the α-MnO2/PS system maintained high catalytic activity across a broad pH range (3.0–9.0). The fastest degradation rate was observed under neutral conditions, with a k value of 0.208 min−1, suggesting that neutral pH was optimal for ROS generation and catalytic performance. This was mainly because the surface properties of α-MnO2 under neutral conditions were optimal for the production and reaction of active species [28]. Additionally, in a neutral environment, faster dissociation and migration rates of reactants, along with the favorable crystal structure and porosity of α-MnO2, enhance the reaction rate [28]. In contrast, under acidic or alkaline environments, side reactions such as excessive oxidation–reduction or neutralization reactions may occur, consuming effective ROS and reducing the degradation efficiency of AO7 [45].
The presence of anions commonly found in aquatic environments can interfere with the generation of free radicals during pollutant degradation [46]. As shown in Figure 5, typical anions such as Cl−, NO3−, CO32−, and SO42−, as well as humic acid (HA), exhibited certain impacts on AO7 degradation in the α-MnO2/PS system. Golshan et al. [47] reported that NO3− consumed SO4•−, 1O2, and •OH generated in the system, converting them into lower redox potential species NO3• and NO2• (Equations (3) and (4)), which inhibited the degradation rate of AO7. Similarly, CO32− also inhibited the degradation effect of α-MnO2/PS system on AO7 by competing with active substances (SO4•−) (Equation (5)) [44]. Excess SO42− could partially inhibit the decomposition of PS, thereby reducing the generation of ROS. Cl− reacted with HSO5−, consuming PS in the system and thereby reducing the number of ROS [48]. Furthermore, Cl− reacted with •OH and SO4•−, forming less reactive chlorine-containing species such as Cl•, Cl2•, and ClOH−, as shown in Equations (6)–(9). The formation of these chlorine-containing species reduced the concentrations of effective oxidative free radicals in the system [49,50].
Furthermore, HA significantly boosted the degradation efficiency of AO7 in the α-MnO2/PS system. HA played a role in the electron transfer processes, accelerating oxidative–reduction reactions and enhancing the overall degradation performance [51].
NO3− + SO4•− → SO42− + NO3•
NO3− + •OH → HO− + NO3•
CO32− + SO4•− → SO42− + CO3•−
2Cl− + HSO5− + H+ → Cl2 + H2O + SO42−
Cl− + SO4•− → SO42− + Cl•
Cl− + Cl• → Cl2•−
Cl− + HO• → SO4•− + ClOH•−

2.4. ROS Responsible for AO7 Degradation

In sulfate-based advanced oxidation systems, the mechanisms are primarily divided into radical and non-radical systems, with the latter further categorized into singlet oxygen, electron transfer, and surface-active species of catalysts [28,31,46]. To investigate the roles of ROS in the α-MnO2/PS system, quenching experiments were conducted using methanol (MeOH), tert-butanol (TBA), furfural alcohol (FFA), and benzoquinone (BQ) as scavengers for •OH, SO4•− and •OH, O2−, and 1O2, respectively [39,51].
Figure 6 illustrates the impacts of various scavenger concentrations on AO7 degradation efficiency. The increase in the MeOH concentration significantly decreased AO7 degradation, indicating that •OH and SO4•− had a significant inhibiting impact on the degradation of AO7. When the MeOH concentration reached 0.20 mM, the AO7 degradation efficiency dropped from 99.7% to 87.3% after 30 min treatment (Figure 6a). Conversely, increasing the concentration of TBA had minimal effects on AO7 degradation. Under the TBA concentration of 0.20 mM, the AO7 degradation efficiency only decreased by 4.24% (Figure 6b). Similarly, when the concentration of BQ reached 0.20 mM, the degradation efficiency of AO7 decreased by only 2.11% (Figure 6c), suggesting that •OH and O2− were not the primary ROS involved in AO7 degradation.
Figure 6d demonstrates the effects of varying FFA concentrations on AO7 degradation. Increasing the FFA concentration led to a decrease in AO7 degradation efficiency. At the FFA concentration of 0.20 mM, the degradation rate of AO7 decreased from 99.7% to 90.7% after 30 min, indicating that 1O2 played an important role in the degradation of AO7. In summary, the quenching experiments indicated that the main ROS included •OH, SO4•−, O2−, and 1O2 in the α-MnO2/PS system, with the degradation of AO7 primarily influenced by SO4•− and 1O2. Dong et al. [28] also found consistent results on the oxidation of bisphenol A by the Fe3O4/α-MnO2-activated persulfate oxidation process.

2.5. Mechanisms of AO7 Degradation

The UV-vis spectra of AO7 at different treatment times are presented in Figure 7a. With increasing the reaction time, the azo bonds in the chromophore groups of AO7 molecules were broken, leading to a gradual decrease in the absorbance peak intensity at 484 nm, eventually approaching zero (Figure 7a). The color of the solution also changed from orange-yellow to colorless. After 5 min of reaction, the absorbance peak at 228 nm significantly increased, while the peak at 310 nm gradually diminished. According to Luo et al. [52], AO7 exhibited characteristic absorbance wavelengths at 228 nm, 310 nm, and 484 nm, corresponding to its phenyl ring, naphthalene ring structures, and azo bond (-N==N-), respectively. These results indicated that the naphthalene ring structure of AO7 was progressively oxidized and decomposed during the reaction, leading to the formation of new compounds containing phenyl ring structures [53].
To investigate the electron transfer process in the α-MnO2/PS system, cyclic voltammetry (CV) and linear sweep voltammetry (LSV) analyses were conducted for both AO7 and AO7/PS systems, using a glassy carbon electrode modified with α-MnO2 as a reference. The electrolyte was a 0.05 mM Na2SO4 solution. As shown in Figure 7b, in the scanning range of −1.0 to 1.0 V, the CV curve of the AO7 system exhibited a pronounced oxidation peak at 0.70 V, indicating a single-electron transfer between α-MnO2 and AO7. In contrast, the CV curve of the AO7/PS system displayed a prominent oxidation peak at 0.74 V and a weaker reduction peak at 0.72 V. The emergence of these new peaks suggested that electron transfer also occurred between AO7 and PS, accompanied by the generation of active free radicals [36].
To assess the stability and reusability of α-MnO2, the residual catalysts after each reaction were collected, thoroughly washed with ultrapure water, dried, and then reused in subsequent AO7 degradation experiments under the same conditions. After five cycles, the AO7 degradation efficiency still reached up to 98.1% (Figure 7c). This demonstrated that the α-MnO2 catalyst exhibited an excellent stability and reusability in this catalytic process [54,55].
Table 1 compares various metal catalytic materials and PMS systems for the removal of organic pollutants. In this study, the catalyst (α-MnO2) dosage was 50 mg/L, and the oxidant concentration was 0.05 mol/L, resulting in a 98% degradation rate of AO7 within 30 min. This system demonstrates an efficient removal rate of Orange II under lower catalyst dosages, lower oxidant concentrations, and neutral conditions compared to existing studies. This indicates that the α-MnO2 material synthesized in this study offers significant advantages in treating organic dye wastewater while requiring less stringent reaction conditions.

3. Material and Methods

3.1. Reagents and Materials

Ammonium persulfate ((NH4)2S2O8), ammonium sulfate ((NH4)2SO4), and manganese sulfate (MnSO4) were purchased from Shanghai Aladdin Biochemical Technology, Co., Ltd. (Shanghai, China). AO7 was purchased from Sinopharm Group Chemical Rematerial, Co., Ltd. (Shanghai, China). Additional reagent information is provided in Table 2. All chemicals were of analytical grade and used as received.

3.2. Synthesis of MnxOy Materials

Manganese oxides were prepared using methods with previously reported optimizations [29]. For α-MnO2 preparation, 0.008 mol of (NH4)2S2O8, 0.008 mol of MnSO4, and 0.02 mol of (NH4)2SO4 were dissolved in 40 mL of deionized water. The solution was then transferred to a reaction flask and heated at 140 °C in a muffle furnace for 12 h to obtain a black precipitate. The precipitate was washed six times with anhydrous ethanol and deionized water, then dried at 80 °C for 16 h. The product was labeled α-MnO2. For β-MnO2 preparation, 0.008 mol of (NH4)2SO4 and 0.008 mol of MnSO4 were dissolved in 40 mL of deionized water at a room temperature under magnetic stirring until homogeneous. The solution was then subjected to a hydrothermal reaction at 140 °C for 12 h. The precipitate was washed five times with anhydrous ethanol and deionized water, then dried at 80 °C for 16 h to obtain β-MnO2. For γ-MnO2 preparation, the procedures were similar with those of β-MnO2, and the hydrothermal reaction temperature was 90 °C. For Mn2O3 preparation, MnO2 was placed in a polytetrafluoroethylene crucible and heated in a muffle furnace at 550 °C for 2 h to convert MnO2 to Mn2O3.

3.3. Degradation of AO7 by MnxOy Materials

The catalytic performances of different MnxOy polymorphs were evaluated using AO7 as a model compound. The detailed experimental procedure is shown in Text S1 in the SI. In a batch experiment, the AO7 concentration was 10 mg/L, and the treatment volume was 50 mL. After treatment, the suspension was analyzed using a UV-visible spectrophotometer (U2800, Shimadzu, Kyoto, Japan) at a wavelength of 484 nm.

3.4. Analytical Methods

Crystal structure and crystallinity of MnxOy were analyzed using a D8 Advance X-ray powder diffractometer (Bruker, Germany) with a scanning range of 10–90° [60]. Chemical structures and crystal types of MnxOy materials were analyzed by X-ray photoelectron spectroscopy (XPS, Thermo Fisher, Waltham, MA, USA) [61]. A CHI 660E Electrochemical Workstation (Shanghai, China), including electrochemical impedance spectroscopy (EIS), linear sweep voltammetry (LSV), and cyclic voltammetry (CV) were used to characterize the electrochemical properties of catalysts [36]. The details are shown in Text S2. Changes in the chemical structure of AO7 during decomposition were analyzed using a Shimadzu UV-vis spectrophotometer (U2800) [62].

4. Conclusions

In this study, α-MnO2, β-MnO2, γ-MnO2, and Mn2O3 were successfully synthesized using the co-precipitation method. These catalysts could activate PS to the degradation of AO7 with degradation efficiencies of 98.60%, 82.47%, 97.15%, and 16.14%, respectively. XPS, XRD, and electrochemical analyses revealed that α-MnO2 possessed an excellent crystal structure and abundant surface vacancies. With the α-MnO2 dosage of 0.075 g, PS concentration of 0.20 mol/L, and AO7 concentration of 10 mg/L, the degradation rate constant was 0.142 min−1. The α-MnO2/PS system demonstrated great catalytic degradation performances for AO7 across a wide pH range of 3.0–9.0. Cl−, CO32−, and NO3− could react with SO4•− to produce less oxidative ROS; excessive SO42− also inhibited the decomposition of PS, leading to a reduced degradation rate of AO7. Radical quenching experiments indicated that the oxidation and degradation of AO7 were primarily driven by •OH, SO4•−, O2−, and 1O2, with SO4•− and 1O2 being the predominant ROS. Electrochemical analysis and full-spectrum UV scanning revealed that these ROS primarily targeted the naphthalene ring structure of AO7, resulting in the formation of new compounds containing a benzene ring. α-MnO2 exhibited an excellent stability and could be effectively reused. This study provided an effective approach for addressing dye wastewater treatment challenges.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/catal14110758/s1: Supporting Information Texts S1 and S2 include degradation experiments and characterization of MnxOy. Figure S1 represents the standard curve of AO7. Table S1. Main materials.

Author Contributions

Conceptualization, T.W. and H.L.; methodology, Y.L.; software, Y.L.; validation, T.W. and H.L.; formal analysis, H.L.; investigation, Y.L.; resources, T.W.; data curation, H.L.; writing—original draft preparation, Y.L. and H.G.; writing—review and editing, Y.L. and H.L.; visualization, T.W. and H.G.; supervision, T.W.; project administration, T.W.; funding acquisition, T.W. and H.L. All authors have read and agreed to the published version of the manuscript.

Funding

The authors thank the National Natural Science Foundation of China (42377388), Ningxia Key Research and Development Program (2023BEG02047, 2024BEG02001), and Natural Science Foundation of Ningxia Province (2023AAC05004).

Data Availability Statement

The original data presented in the study are included in the article.

Conflicts of Interest

The author, Yujuan Li, is employed by Ningxia Academy of Environmental Sciences, Co., Ltd. The remaining authors declare that there are no business or financial relationships that could be perceived as potential conflicts of interest in the course of the study.

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Catalysts 14 00758 g001
Figure 1. Enhanced degradation of AO7 using persulfate activated by different manganese oxides: efficiency, kinetic constants, and adsorption analysis ((a) degradation efficiency, (b) kinetic constants, (c) adsorption efficiency).
Figure 1. Enhanced degradation of AO7 using persulfate activated by different manganese oxides: efficiency, kinetic constants, and adsorption analysis ((a) degradation efficiency, (b) kinetic constants, (c) adsorption efficiency).
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Figure 2. Comprehensive electrochemical properties of catalysts ((a) cyclic voltammetry (CV), (b) linear sweep voltammetry (LSV), (c) electrochemical impedance spectroscopy (EIS)).
Figure 2. Comprehensive electrochemical properties of catalysts ((a) cyclic voltammetry (CV), (b) linear sweep voltammetry (LSV), (c) electrochemical impedance spectroscopy (EIS)).
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Catalysts 14 00758 g003
Figure 3. Comprehensive characterization of α-MnO2 ((a) XRD analysis of α-MnO2, (b) full survey spectrum, (c) high-resolution Mn 2p core level spectrum, (d) high-resolution O 1s core level spectrum).
Figure 3. Comprehensive characterization of α-MnO2 ((a) XRD analysis of α-MnO2, (b) full survey spectrum, (c) high-resolution Mn 2p core level spectrum, (d) high-resolution O 1s core level spectrum).
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Figure 4. Degradation performance and kinetic evolution of AO7 under varied conditions ((a,b) effect of α-MnO2 dosage, (c,d) influence of PS concentration, (e,f) impact of pH on degradation efficiency).
Figure 4. Degradation performance and kinetic evolution of AO7 under varied conditions ((a,b) effect of α-MnO2 dosage, (c,d) influence of PS concentration, (e,f) impact of pH on degradation efficiency).
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Figure 5. Influence of common anions on AO7 degradation (conditions: 0.1 mM anions; 5 mg/L HA; 0.075 g α-MnO2: 0.2 mol/L PS; 10 mg/L AO7).
Figure 5. Influence of common anions on AO7 degradation (conditions: 0.1 mM anions; 5 mg/L HA; 0.075 g α-MnO2: 0.2 mol/L PS; 10 mg/L AO7).
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Figure 6. Impact of various scavengers on AO7 degradation performance via manganese oxide-activated persulfate ((a) methanol, (b) tert-butyl alcohol, (c) para-benzoquinone, (d) furfuryl alcohol. Conditions: 0.1 mM anions; 5 mg/L HA; 0.075 g α-MnO2: 0.2 mol/L PS; 10 mg/L AO7).
Figure 6. Impact of various scavengers on AO7 degradation performance via manganese oxide-activated persulfate ((a) methanol, (b) tert-butyl alcohol, (c) para-benzoquinone, (d) furfuryl alcohol. Conditions: 0.1 mM anions; 5 mg/L HA; 0.075 g α-MnO2: 0.2 mol/L PS; 10 mg/L AO7).
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Figure 7. Mechanisms of AO7 degradation and stability assessment of catalytic materials ((a) UV-vis spectra, (b) CV curves, (c) stability of α-MnO2. α-MnO2 concentration: 0.075 g; PS concentration: 0.2 mol/L; AO7 concentration: 10 mg/L).
Figure 7. Mechanisms of AO7 degradation and stability assessment of catalytic materials ((a) UV-vis spectra, (b) CV curves, (c) stability of α-MnO2. α-MnO2 concentration: 0.075 g; PS concentration: 0.2 mol/L; AO7 concentration: 10 mg/L).
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Table 1. Comparison of removal of dye pollutants by different catalytic materials/PMS systems.
Table 1. Comparison of removal of dye pollutants by different catalytic materials/PMS systems.
SystemConcentrationConditionDegradation RateReferences
Co/MgO/PMS50 mg/LCatalyst = 150 mg/L
PMS: AO7 = 10:1,
pH = 4.0		>99%
10 min		[56]
CuFe2O4+HCO3/
PMS		100 mg/LPMS = 0.98 mmol/L
HCO3 = 2 mmol/L
pH = 5.9 ± 0.1		95 %
30 min		[57]
Fe(II)/PMS25 μMCatalyst = 30 μM
PMS = 1.5 mM
pH = 3.15		39%
6 min		[58]
ZVC/PMS25 μMCatalyst = 0.3 g/L
PMS = 1.5 mM
pH = 3.15		45%
10 min
Fe(II)/ZVC/PMS25 μMMS = 1.5 mM
pH = 3.15		96%
10 min
CoMg2Mn-LDO100 mg/LCatalyst = 90 mg
PMS = 100 mg
pH = 3.0		97%
15 min		[59]
α-MnO250 mgCatalyst = 50 mg
PMS 0.05 mol/L
pH = 3.0		98.3%
15 min		This study
Table 2. Summary of key materials used in the study.
Table 2. Summary of key materials used in the study.
ReagentsMolecular FormulaGradeManufacturer
Ammonium persulfate(NH4)2S2O8ARShanghai Aladdin, Co., Ltd., Shanghai, China
Ammonium sulfate(NH4)2SO4AR
Manganese sulfateMnSO4·H2OAR
Orange IIC16H11N2NaO4SAR
Sodium persulfateNa2S2O8AR
Methyl alcoholCH3OHAR
Tertiary butanolC4H10OAR
Furfuryl alcoholC5H6O2AR
Sodium nitrateNaNO3AR
Humic acidC64O26H55N4AR
Furfuryl alcoholC5H6O2AR
p-benzoquinoneC6H4O2ARShanghai Maclin Biochemical Technology, Co., Ltd., Shanghai, China
Sodium chlorideNaClAR
Anhydrous sodium sulfateNa2SO4ARSinopharm Group Chemical Reagent, Co., Ltd., Shanghai, China
Sodium carbonate anhydrousNa2CO3AR
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Li, Y.; Guo, H.; Li, H.; Wang, T. Performance of Dye-Containing Wastewater Treatment Using MnxOy-Catalyzed Persulfate Oxidation. Catalysts 2024, 14, 758. https://doi.org/10.3390/catal14110758

AMA Style

Li Y, Guo H, Li H, Wang T. Performance of Dye-Containing Wastewater Treatment Using MnxOy-Catalyzed Persulfate Oxidation. Catalysts. 2024; 14(11):758. https://doi.org/10.3390/catal14110758

Chicago/Turabian Style

Li, Yujuan, He Guo, Hu Li, and Tiecheng Wang. 2024. "Performance of Dye-Containing Wastewater Treatment Using MnxOy-Catalyzed Persulfate Oxidation" Catalysts 14, no. 11: 758. https://doi.org/10.3390/catal14110758

APA Style

Li, Y., Guo, H., Li, H., & Wang, T. (2024). Performance of Dye-Containing Wastewater Treatment Using MnxOy-Catalyzed Persulfate Oxidation. Catalysts, 14(11), 758. https://doi.org/10.3390/catal14110758

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